The present invention relates to a resin-coated Alxe2x80x94Zn alloy coated steel sheet, which is excellent in formability, resistance to chromium dissolution, corrosion resistance, alkali resistance, and paintability.
In general, Alxe2x80x94Zn (aluminum-zinc) alloy coated steel sheets can be produced by plating an alloy having a composition of 4 to 75 wt % of Al, small amount of Si (silicon), Mg (magnesium), Ce (cerium)-La (lanthanum) or the like, and the balance of Zn on a steel substrate. As commercially available coated steel sheets, there are two kinds of a low Alxe2x80x94Zn alloy coated steel sheet with an alloy coating layer having a composition of 4 to 10 wt % of Al, small amount of Ce-La and the balance of Zn, and a high Alxe2x80x94Zn alloy coated steel sheet with an alloy coating layer having the composition of 55 wt % of Al, 43.4 wt % of Zn and 1.6 wt % of Si. When a coating thickness of the low Alxe2x80x94Zn alloy coated steel sheet is equal to that of a conventional hot dip galvanized steel sheet, the corrosion resistance of the low Alxe2x80x94Zn alloy coated steel sheet is 1.5 to 2 times as high as that of the hot dip galvanized steel sheet. In addition, when the coating thickness of the high Alxe2x80x94Zn alloy coated steel sheet is equal to that of the conventional hot dip galvanized steel sheet, the corrosion resistance of the high Alxe2x80x94Zn alloy coated steel sheet is 3 to 6 times as high as that of the hot dip galvanized steel sheet. In particular, the highxe2x80x94Alxe2x80x94Zn alloy coated steel sheet exhibits excellent heat resistance and thermal reflectivity.
The composition of the alloy coating layer of this high Alxe2x80x94Zn alloy coated steel sheet is determined to provide good balance between a passivation-film protecting action of Al and a sacrificial anticorrosive action of zinc. In addition, unlike the hot dip galvanized steel sheet, the alloy coating layer has a structure that aluminum-rich phases are surrounded with zinc-rich phases in a network-like manner. Immediately after corrosion begins, a dense, stable compound is generated to fill the network-like space, so that a further progress of corrosion can be prevented. It is believed that excellent corrosion resistance is achieved by this mechanism. Moreover, as described above, since the high Alxe2x80x94Zn alloy coated steel sheet is excellent in the heat resistance and thermal reflectivity, it is becoming pervasive as architectural materials for roof and wall, construction materials for guardrail, soundproofing material, fence for protection from snow and drain ditch, materials for automobile, household electrical appliance and industrial equipment, and a substrate for painted steel sheet.
Although the corrosion resistance of the above described coated steel is excellent, it means that the time that elapses before red rust occurs from corrosion of iron of the steel substrate is long. Therefore, when a passivation is not treated on the coated surface, white rust or black rust will occur in a short time. As a result, a beautiful silver-white appearance of the coated steel sheet will be lost.
For that, a chromate treatment for preventing the occurrence of white rust or black rust, method of coating a resin film by use of a composition containing hexavalent chromium in a water-base resin having an acid value of 10 to 200, which is disclosed in Japanese Patent Publication No. 4-2672, or a treatment of coating a silicone resin containing a lubricating material with a small amount of chromium, which is disclosed in Japanese Patent Early Publication No. 7-251128, have been adopted.
Although the corrosion resistance of the high Alxe2x80x94Zn alloy coated steel sheet can be improved by the chromate treatment, the hardness of the alloy coating layer is too high because of the large Al content, so that there are some problems when forming the coated steel sheet by roll forming or stamping. For example, the alloy coating layer may often receive damages because of poor lubrication between the high Alxe2x80x94Zn alloy coated steel sheet and a roll or a stamping die. Alternatively, when the alloy coating layer is partially melted by friction therebetween, the melted alloy may adhere to the roll or the stamping die. In addition, there is a problem of fine metal particles resulting from the high Alxe2x80x94Zn alloy coated steel sheet during the roll-forming or stamping operation. When the metal particles adhere to corner portions of the rolled or stamped article, a seizing-up phenomenon, scratches or abrasion may occur. These result in a deterioration of the appearance of the formed article.
On the other hand, although the occurrence of bad articles in the roll-forming or stamping operation can be prevented by the surface treatment disclosed in Japanese Patent Publication No. 4-2672, there are another problems described bellow. That is, since the resin film formed by the surface treatment contains hexavalent chromium ion to maintain the corrosion resistance, condensation may occur on the resin-coated Alxe2x80x94Zn alloy coated steel sheet, or a dissolution of the hexavalent chromium ion from the resin film may occur when the resin film is exposed to rain for a long time period. As a result, there is a possibility of causing environmental pollution. In addition, when a contact between the resin film and an alkaline material such as mortar or concrete is kept for a long time period, discoloration to black peculiar to aluminum may be caused on the resinxe2x80x94coated Alxe2x80x94Zn alloy coated steel sheet. Consequently, a deterioration of the appearance of the formed article occurs.
A resin-coated Alxe2x80x94Zn coated steel sheet formed according to the method disclosed in Japanese Patent Early Publication No. 7-251128 exhibits excellent formability because the resin composition used contains a lubricating agent and also good corrosion resistance even after forming. However, when a painting is formed on the resin-coated Alxe2x80x94Zn coated steel sheet, there is a possibility of a poor adhesion between the resin film and the painting because the resin film hardly contain functional group capable of bonding with the painting.
In addition, when the resin-coated Alxe2x80x94Zn alloy coated steel sheet is produced on a high-speed galvanizing line, in which a surface treatment is performed immediately after a hot dip Alxe2x80x94Zn alloy is coated on a steel substrate, a resin composition is applied on the hot dip Alxe2x80x94Zn alloy coated steel sheet in the surface treatment, and then dried at a relatively low temperature of 60 to 120xc2x0 C. for a short time period, i.e., 3 to 15 seconds. However, when using the resin composition of the prior art, there is a problem that it is difficult to stably provide a high-quality resin film.
Therefore, the present invention is directed to a resin-coated Alxe2x80x94Zn alloy coated steel sheet capable of providing the following advantages:
(1) It is possible to prevent a deterioration of the appearance of the resin-coated Alxe2x80x94Zn alloy coated steel sheet by forming, for example, roll forming or stamping.
(2) Even when a drying operation for the formation of a resin film is performed at a low temperature for a short time period on the conventional high-speed galvanizing line, it is possible to stably provide the resin film having a suitable formability.
(3) Chromium dissolution hardly occurs even when the resin film is exposed to damp air.
(4) The resin-coated Alxe2x80x94Zn alloy coated steel sheet is excellent in corrosion resistance, i.e., resistance to white rust, resistance to black rust, and alkali resistance.
(5) It is possible to provide an improved adhesion between the resin film and a painting formed on the resin film, if necessary.
That is, the resin-coated Alxe2x80x94Zn alloy coated steel sheet of the present invention is composed of an Alxe2x80x94Zn alloy coated steel sheet as a substrate and a resin film formed on the substrate by use of a chromate containing resin composition. The resin-coated Alxe2x80x94Zn alloy coated steel sheet of the present invention is produced according to the following method. That is, (A) a silane coupling agent having amino group, (B) chromium ion, and (C) at least one alcohol selected from the group consisting of trihydric alcohol and dihydric alcohol having the number of carbon of 2 to 3 are compounded into (D) an acrylic polymer resin emulsion including carboxyl group and glycidyl group and having an acid value of 10 to 60. A pH of the resultant mixture is adjusted within a pH range of 7 to 9 to obtain the chromate containing resin composition. After the chromate containing resin composition is applied on the substrate to form an applied film, the applied film is dried to obtain the resin film. In this method, it is essential that a compounding amount of the silane coupling agent (A) is within a range of 0.5 to 3.0 wt % with respect to a resin solid component of the acrylic polymer resin emulsion (D), and a compounding amount of the alcohol (C) is within a range of 25 to 150 wt % with respect to the chromium ion (B). The resin film formed according to the above method is characterized in that an amount of the resin film is within a range of 0.5 to 3.0 g/m2, and a content of the chromium ion (B) in the resin film is within a range of 5 to 50 mg/m2.
In a preferred embodiment of the present invention, a mole ratio of carboxyl group:glycidyl group in the acrylic polymer resin emulsion (D) is 1: 0.3 to 3.0.
In a further preferred embodiment of the present invention, an amount of the chromium ion (B) in the chromate containing resin composition is within a range of 0.5 to 2.0 wt % with respect to the resin solid component of the acrylic polymer resin emulsion (D).
These and still other objects and advantages will become apparent from the following detail description of the preferred embodiments and examples of the invention.